1. Field of the Invention
The present invention relates to a cell search method, a communication synchronization apparatus, a portable terminal apparatus, and a recording medium storing a program for realizing the method and the apparatus by a function of software and, more particularly, to a method and an apparatus suitably used, e.g., to establish synchronization between a mobile communication terminal such as a portable telephone and a base station.
2. Description of the Related Art
Conventionally, in analog FDMA (Frequency Division Multiple Access) for connecting mobile stations such as portable telephones to one base station using different frequencies, one frequency band is exclusively used for communication by one mobile station. This decreases the utilization efficiency of the divided frequency bands and also makes it impossible to increase the number of users in the service area (cell) of the base station.
Currently, digital TDMA (Time Division Multiple Access) for time-divisionally connecting one frequency band to mobile stations is often used in place of FDMA. According to this scheme, since two or more mobile stations can be assigned to one frequency band for communication, the number of users can be increased as compared to FDMA.
In TDMA, however, since segmented signals are time-divisionally exchanged between the base station and the mobile stations, the amount of information of communication by one mobile station becomes small. To increase the amount of information of communication, current digital portable telephones and the like transmit signals compressed by encoding. On the receiving side, the signal is expanded and reproduced. For this reason, the quality of reproduced speech degrades.
In recent years, CDMA (Code Division Multiple Access) using direct spread spectrum has received a great deal of attention as a communication scheme capable of greatly increasing the utilization efficiency of each frequency band and also reproducing high-quality speech.
In CDMA, signals to be transmitted from a base station to mobile stations are spread using spreading codes unique to the respective mobile stations, and transmitted using one frequency band. A receiving-side mobile station multiplies received signals by the specific spreading code assigned to the mobile station, to calculate correlation between it and each spreading code used on the transmitting side. The mobile station thereby detects the peak value of correlation, and extracts only a signal addressed to the mobile station. According to CDMA, one frequency band can be assigned to a larger number of mobile stations by using different spreading codes. In addition, since the amount of information to be transmitted can be increased, the quality of reproduced speech can also be improved.
When a mobile station such as a portable telephone is powered on, it must receive a predetermined message from the base station in the area (cell). In CDMA, the message from the base station is repeatedly sent in units of predetermined slots, as shown in FIG. 1. As indicated by an arrow in FIG. 1, the mobile station is not always powered on at the start timing of a slot and cannot correctly read the message when it is powered on at another timing.
To decode properly the message contained in the slot, the start timing of the slot must be detected (this is called “cell search”), and the message must be received from that timing. Cell search is not limited to the above-described initial cell search for catching the cell to be connected at the time of powering the mobile station on. More specifically, even after powering on, for example, when the mobile station moves across cells, synchronization may shift. Hence, synchronization shift is always monitored by periodically performing cell search.
FIG. 2 is a block diagram showing the construction of a cell search circuit of conventional wideband CDMA communication scheme (direct spread CDMA), which is provided in a mobile station. Referring to FIG. 2, for a reception signal (transmission channel signal as shown in FIG. 1, which is transmitted from a base station (not shown)), the first 1-bit data of each slot, which is indicated by a hatched portion in FIG. 1, is spread by a common spreading code (a spreading code that changes 256 times in one bit: number of chips=256) prepared independently of the spreading codes unique to the respective mobile stations. Normally, such a transmission channel signal for cell search is transmitted using a common channel (perch channel).
The in-phase component I and the quadrature component Q of the voltage of such a reception signal are converted into digital signals by an A/D converter 101 and sequentially supplied to a correlator 102 such as a matched filter or a sliding correlator, in units of slots (one slot corresponds to 10 symbols) from the power-on timing of the mobile station. The correlator 102 integrates each digital signal input from the A/D converter 101 with the spreading code common to the mobile stations, which is generated by a code generator 103, so as to perform despreading.
The in-phase component I and the quadrature component Q of the voltage, which are output from the correlator 102, are supplied to a power conversion section 104 and converted into power values in units of predetermined sampling points in the slot. The power values obtained at the sampling points are sequentially stored at addresses of a memory (RAM) 107 corresponding to the respective sampling points, through an adder 106 in a power value integration section 105.
In the above process, only the portion with large correlation with the common spreading code multiplied by the mobile station in the first slot after powering the mobile station, i.e., only the power value of the hatched portion in FIG. 1 where the common spreading code is multiplied by the base station (not shown) appears as a peak. Hence, when this peak portion is detected, the start position of the slot can be confirmed, and subsequent communication can be performed in accordance with that timing.
In fact, a mobile station receives transmission channel signals with delays from two or more base stations near the mobile station, as shown in FIG. 1. In addition, signals from one base station include not only direct waves directly received from the base station but also waves reflected by buildings or the ground and then received. For this reason, a received transmission channel signal has, in one slot, a number of portions spread by the common code, and a number of peak power values are detected in one slot. Besides, when the mobile station moves in the cell search operation, the peak in the next slot may be detected at a position different from the previous position.
In consideration of these situations, the peak power value is detected not for only the first slot after powering the mobile station but over several slots. More specifically, the power integration values up to the preceding slot are read out from the RAM 107 in units of sampling points and supplied to the adder 106. The power values at the same sampling point in the current slot are added and stored in the RAM 107 again. By integrating the power values over several slots, the portion with the largest peak is finally recognized as the start portion of the transmission channel signal sent from the closest base station.
The number of times of integration of the power values (slot count) is set in an integration count setting register 108. A counter 109 increments the count value by one, every time integration of one slot is ended. When the count value reaches a value set in the integration count setting register 108, the counter 109 outputs a time-out signal, and integration is ended.
However, when cell search is performed using the above conventional method, the RAM 107 for storing the power integration value at each sampling point requires the capacity of 10,240 words. That is, the number of chips (the number of cycles) in one slot of the perch channel for cell search is 256×10=2560. To increase the accuracy of peak value detection, one chip is divided into four divisions, and oversampling of 4 times is performed. Hence, the total number of sampling points in one slot is 10,240 (when the chip rate is 4 Mcps).
The area of the RAM 107 for storing power integration values corresponding to 10,240 words is several mm square or more. This causes a very large circuit area. In particular, for a portable communication terminal such as a portable telephone, it is important to make it compact and lightweight. Circuits for transmission, reception, and cell search functions need be stored in one chip. However, since the ratio of the cell search circuit to the LSI becomes very high, the LSI itself cannot be made compact.
In addition, since data having the largest value must be selected from 10,240 power integration values stored in the RAM 107, the processing load is heavy, and it takes a long time to complete cell search. For example, a long time is required for initial cell search upon powering on, and the rise time until communication is enabled, becomes very long.
In cell search using the above conventional method, the number of times of integration of power values is set to be relatively large (e.g., for 32 slots) in consideration of a bad reception sensitivity state such that the path having the peak can be extracted even when the signal reception sensitivity is low. For this reason, the time required for cell search is constant independently of the signal reception state. Even when the reception state is good, integration is performed a number of times more than necessity, and it takes a long time to complete cell search.